The Ruger SR9 uses a proprietary magazine.
It doesn't.
Try e-gunparts.com or Mec Gar
I paid $439 for mine at Outdoor World. MSRP is 525
Can't be answered without a DETAILED description of ALL markings, to include label on box.
I have seen very few used SR9 Rugers at my local gun shops. However you can buy a brand new one for around $360.00. That is a pretty good deal if they get some of the bugs worked out.
From the Ruger website: What type of ammunition should I use in my Ruger 9mm pistol? The Ruger 9mm pistols are chambered for the 9x19mm NATO Parabellum (9mm Luger) cartridge, compatible with the U.S. and foreign military or commercial 9x19mm loads manufactured in accordance with NATO, U.S., SAAMI, or CIP standards, including high-velocity, subsonic, tracer, hollow point, ammunition loaded in aluminum, steel, or brass cartridge cases, +P and +P+ ammunition.
Depends entirely on which 357, and the condition of both guns. Some are worth more, some less.